Phosphor coating for arc discharge lamps

ABSTRACT

A small quantity of benzoic acid is added to a phosphor coating suspension in order to prevent a loss in brightness of lamps made therefrom.

States atent Taubner et al.

PHOSPHOR COATING FOR ARC DISCHARGE LAMPS Fred R. Taubner, Danvers, Mass;Charles F. Chenot, Towanda, Pa.

Assignee: Sylvania Electric Products Inc. Filed: June 30, 1969 Appl.No.: 837,939

Inventors:

U.S. CL ..1 17/335 L, 252/3013 Int. Cl. ..H0lk 1/28 Field ofSearch..252/301.3;117/33.5 R, 33.5 E,

117/335 CM, 33.5 L; 96/36.1

[ 1 Mar. 14, 1972 Primary Examiner-Alfred L. Leavitt AssistantExaminer-Wayne F. Cyron Art0rneyNorman J. OMalley and JamesTheodosopoulos [5 7] ABSTRACT A small quantity of benzoic acid is addedto a phosphor coating suspension in order to prevent a loss inbrightness of lamps made therefrom.

7 Claims, No Drawings BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates generally to are discharge lamps andparticularly to phosphor coatings used in such lamps.

2. Description of the Prior Art In the manufacture of arc dischargelamps having a phosphor on the inner wall of the lamp bulb, the phosphoris generally deposited on the bulb from a suspension of the powderedphosphor in a suitable liquid vehicle. For example, in the manufactureof a fluorescent lamp, the open-ended lamp bulb is positioned verticallyand the phosphor suspension is introduced into the top end of the bulband caused to flow down the entire inner surface thereof. A sufficientamount of the suspension adheres to the inner wall to form a uniformlayer of wet phosphor thereon. The solvents are then evaporated and thephosphor coated bulb is then stored until it can be processed into afinished fluorescent lamp. The duration of storage time of coated bulbsprior to processing depends on exhaust machine production schedules inaddition to other manufacturing requirements and can vary from aboutone-half hour to 16 hours or even more. Normally, the length of storagetime has little effect on the luminescent properties of finishedfluorescent lamps.

However, in some phosphor suspensions, aliphatic amines are added forthe purpose of stabilizing the suspension by promoting the dispersion ofthe phosphor particles. The use of these amines results in a phosphorcoating that is very reactive with acidic atmospheric gases,particularly sulphur dioxide gas. A reaction therebetween can occur whenthe concentration ofthis gas in the atmosphere is as low as 0.05 to 0.10parts er million, such a concentration being frequently exceeded in theatmosphere ofthe storage area of phosphor coated bulbs.

The reaction between the amine dispersant and the sulphur dioxideproduces a material that will not completely burn off during lamp bakingprocesses. As a result the brightness of fluorescent lamps madetherefrom is decreased, and the amount of decrease is generallyproportional to the degree of sulphur dioxide contamination of thephosphor. In extreme cases the decrease in brightness is as high asfifty percent after only a few hundred hours oflamp operation.

SUMMARY OF THE INVENTION We have discovered that the reaction between anamine dispersant of a phosphor coating and sulfur dioxide can beinhibited by the addition of a small quantity of benzoic acid to thecoating suspension. The benzoic acid can be premixed with the amineprior to formulating the phosphor coating suspension or it can be addedto the prepared phosphor coating suspension containing the amine.However, the benzoic acid should not be allowed to come into contactwith the phosphor suspension before the amine has been added in order tobe effective in inhibiting the sulphur dioxide-amine reaction.

The deleterious effects of sulphur dioxide on fluorescent phosphorcoated bulbs and on the corresponding light output depreciation oflampsmade therefrom are reduced in proportion to the amount of benzoic acidaddition, with substantially no brightness loss occurring at optimumbenzoic acid concentrationsv An additional advantage resulting from thecombination of benzoic acid with an amine dispersant in a phosphorcoating suspension is a reduction in the amount of phosphor required toobtain the optimum thickness of phosphor coating on the bulb wall. Thisdecrease in the amount of phosphor results in a three to four percentsavings in phosphor material costs, which is substantial since thephosphor is one of the most expensive material components ofafluorescent lamp.

In addition to inhibiting the sulphur dioxide-amine reaction andreducing the phosphor weight per lamp, the benzoic acid in combinationwith the amine also improves the physical appearance of the bulbphosphor coating by giving a smoother texture thereto in comparison to acoating that does not contain benzoic acid.

Preferably the amount of benzoic acid that is added to a phosphorsuspension is about equal to the weight percentage of the aminedispersant in the suspension, and is less than about 1 or 2 percent ofthe weight of the phosphor.

DESCRIPTION OF THE PREFERRED EMBODIMENT A phosphor coating suspension inaccordance with this invention comprised a calcium halophosphatephosphor activated by antimony and manganese. The phosphor coatingsuspension was prepared by dispersing the phosphor particles in anorganic system consisting of ethyl cellulose as a binder, xylol andbutanol as solvents, and dibutyl phthalate as a plasticizer. Thesuspension was stabilized by the addition of a primary aliphatic aminein the amount of 0.75 percent by weight of the phosphor. The same amountof benzoic acid, 0.75 percent by weight of the phosphor, was then addedin order to render the amine non-reactive to the gaseous sulphur dioxidein the atmosphere.

The phosphor suspension was applied in the usual manner of causing thesuspension to flow down the inner surface of the bulb and allowingsufficient time for the solvents to evaporate, leaving the ethylcellulose, dibutyl phthalate, amine-benzoic acid combination andphosphor particles adhered to the dried bulb.

Control lamp bulbs were phosphor coated by the same procedure exceptthat the benzoic acid was omitted from the coating suspension.

Both groups of bulbs, that is, those containing the benzoic acidadditive and the control without benzoic acid, were subjected to anatmosphere which contained a concentration of from 1 to 2 parts permillion of sulphur dioxide gas for a period ofl hour.

The bulbs were than baked in a lehr in the usual manner at a temperatureof about 600 C. for about two minutes in order to burn off the organiccomponents, leaving the phosphor particles adhered to the inner surfaceof the bulb walls. After baking, the bulbs were processed intofluorescent lamps by the usual lamp manufacturing techniques, and werethen photometered for light output in a standard photometric sphere.After subsequent hundred hour operating intervals, the lamps were againphotometered in order to compare the lamp brightness of the benzoic acidlamps with the lamps which did not have benzoic acid in the phosphorcoating.

The photometric results expressed in lumen units of light output showed3166 lumens after operating hours for the lamps containing benzoic acidin the coating that was exposed to sulphur dioxide gas. The lamps whichdid not contain benzoic acid in the phosphor coating but which were alsoexposed to sulphur dioxide gas measured 1691 lumens after operating 100hours. These results show that the presence of a small quantity ofbenzoic acid renders the phosphor coating which contains the aminestabilizing agent non'reactive to an atmosphere containing sulphurdioxide contamination.

Several concentrations of benzoic acid additions to the phosphor coatingsuspension were evaluated in the same manner and the results of 40 wattlamps made therefrom are listed in Table I. In each case the weight ofamine dispersant in the coating suspension was 0.75 percent of thephosphor weight, the phosphor being a cool white calcium halophosphate.The lamps designated as B were those in which the coated bulbs had beenexposed to sulphur dioxide and the lamps designated as A" were controlswhich had not been exposed to sulphur dioxide. The benzoic acid isexpressed as a percentage of the weight of the phosphor in thesuspension.

TABLE I Lurnens l. Lamp A no benzoic acid 3.180 3,152 Lamp 8 no benzoicacid 1.691 [,7 l7 2. Lamp A 0.157: benzoic acid 3.182 3.136 Lamp 80.157: henzoic acid 2.05? 2,078 3. Lamp A 0.375% benzoic acid 3.1693.]50 Lamp 8 0.37592 benzoic acid 2,980 2,956 4. Lamp A 0.50% benzoicacid 3,181 3.164 Lamp 8 0.50% benzoic acid 3, I 3,088 5. Lamp A 0.75%benzoic acid 3,176 3.153 Lamp B 0.75% benzoic acid 3,166 3,152

It can be seen from test No. l in Table 1 that without benzoic acid inthe coating suspension, sulphur dioxide exposure reduced lamp brightnessby 47% after 100 hours of lamp operation and 46 percent after 200 hours.In test No. 2, where the concentration of benzoic acid was onlyone-fifth that of the amine dispersant, the same brightness reductionwas only 35 percent and 34 percent respectively. And in test No. 5,where the concentration of benzoic acid equalled that of the amine,there was no significant brightness reduction resulting from exposure ofthe coated bulb to sulphur dioxide.

It is likely that some benzoic acid derivatives may also be effective ininhibiting the reaction between an amine dispersant and sulfur dioxidewithout impairing the effectiveness of a phosphor suspension. Forexample, benzoic acid anhydride does inhibit the reaction. But thetexture of a phosphor layer deposited from a phosphor suspensioncontaining the anhydride is not as smooth as that obtained from thebenzoic acid suspension. And, consequently, the economy in the amount ofphosphor required for an optimum coating thickness is less with theanhydride than with the acid.

We claim:

1. A phosphor coating suspension comprising: a phosphor dispersed in aliquid vehicle, said vehicle comprising a binder and a solvent; an aminedispersant aiding in suspending said phosphor; and a small quantity ofbenzoic acid or benzoic acid anhydride.

2. The suspension of claim 1 wherein the weight percent of said benzoicacid is about equal to the weight percent of said amine.

3. The suspension of claim 1 wherein said dispersant is an aliphaticamine.

4. The suspension of claim 1 wherein the weight of said amine dispersantis less than 2 percent of the weight of said phosphor.

5. In the manufacture ofa fluorescent lamp, the steps which comprise:preparing a suspension including a phosphor, an amine dispersant,benzoic acid and a solvent; coating the inner wall of an open ended lampbulb with said suspension; evaporating said solvent from said coating;firing said coated bulb at an elevated temperature to burn off theorganic materials in said coating; and processing said coated bulb intoa finished fluorescent lamp.

6. The process of claim 5 including the step of premixing said amine andsaid benzoic acid prior to the addition thereof to said suspension.

7. The process of claim 5 wherein the weight percent of said amine basedon the weight of said phosphor is about equal to the weight percent ofsaid benzoic acid.

2. The suspension of claim 1 wherein the weight percent of said benzoicacid is about equal to the weight percent of said amine.
 3. Thesuspension of claim 1 wherein said dispersant is an aliphatic amine. 4.The suspension of claim 1 wherein the weight of said amine dispersant isless than 2 percent of the weight of said phosphor.
 5. In themanufacture of a fluorescent lamp, the steps which comprise: preparing asuspension including a phosphor, an amine dispersant, benzoic acid and asolvent; coating the inner wall of an open ended lamp bulb with saidsuspension; evaporating said solvent from said coating; firing saidcoated bulb at an elevated temperature to burn off the organic materialsin said coating; and processing said coated bulb into a finishedfluorescent lamp.
 6. The process of claim 5 including the step ofpremixing said amine and said benzoic acid prior to the addition thereofto said suspension.
 7. The process of claim 5 wherein the weight percentof said amine based on the weight of said phosphor is about equal to theweight percent of said benzoic acid.